EMI: Rework text stack handling

The text stack in EMI must be saved and restored in save games because
the game will call PopText after returning from the menu. Instead of
managing separate lists for each text stack level this patch adds a
_stackLevel field that is incremented for PushText and decremented for
PopText. Only text objects from the stack top (level 0) are rendered.
Merge pull request #773 from inguin/emi-text-stack

EMI: Rework text stack handling